The invention relates to power FETs (field effect transistors), and more particularly to a plurality of FETs stacked in series and capable of handling bidirectional current, for AC application.
Power FETs are known in the art. A FET is unidirectional and conducts current from one main terminal to another in response to gate drive on a third terminal. This three terminal arrangement is widely accepted, and is compatible with standard circuit applications.
The stacking of power FETs in unidirectional applications is also known. Stacking is the interconnection of multiple devices in configurations that result in capabilities beyond those of a single device. The stacking of mulitple power FETs in series results in higher voltage capability, and a better ratio of ON resistance to breakdown voltage. For example, connecting a pair of 100 volt devices in series results in a total voltage capability of 200 volts. The ON resistance in an individual power FET is proportional to the blocking voltage raised to the 2.6 power. Thus, doubling the blocking voltage in a single device would result in an ON resistance which is increased more than six times. Stacking of a pair of devices affords the increased voltage blocking capability but with lower ON resistance. Various problems encountered in stacking include voltage isolation, and differing gate triggering levels. Each of the gates wants to reference to a different level, but it is desirable to drive all the gates from the same source via a single gate terminal.
In order to control a load driven by an AC power source, a plural FET arrangement must be bidirectional, i.e. pass current in both directions. It is desirable that the plural FET circuit be a three terminal device which is compatible with most packaging environments.
The present invention addresses and solves the need for AC voltage capability in a series stacked plural FET arrangement. The FETs are bidirectionally stacked source to source, and have particularly simple and effective drain-referenced common gating circuitry.
A FET is driven into conduction by charging its gate to source capacitance, and thus gating circuitry is usually referenced to the FET source, not the drain. Stacking of power FETs source to source in series relation may make the FET sources difficult to access for gate referencing purposes, or may make source-referencing otherwise difficult or undesirable. The present invention is particularly beneficial where drain-referencing is needed or otherwise desirable for a plurality of power FETs stacked source to source in series.